1. Field of the Invention
This invention relates to a method of manufacturing a welded can body, and more particularly, to a method of manufacturing a welded can body composed of tinplate or a tin-iron alloy plated steel sheet having a welded portion of which the iron-exposed portion is covered with a tin layer to improve the corrosion resistance of the welded portion.
2. Description of the Prior Art
Tinplate is a sheet of low carbon steel having a surface covered with a layer of metallic tin and a layer of tin-iron alloy formed between the metallic tin layer and the steel substrate. In some special cases, a steel sheet having a surface covered with metallic tin without the tin-iron alloy layer is called tinplate. A tin-iron alloy plated steel sheet is designated herein as a low carbon steel sheet having a surface thereof composed mainly of a very thin layer of a tin-iron alloy which is sometimes inevitably covered thereon with a further thinner layer of metallic tin, as disclosed, for example, in Japanese Laid-Open Patent Specification No. 23833/1978. Such tin-iron alloy plated sheet is inexpensive, as it is plated with only a very small quantity of tin. The tin-iron alloy layer, which is composed essentially of FeSn.sub.2, has excellent lacquer adhesion and a higher degree of workability than a layer of metallic chromium on tin free steel, and thus it is less liable to crack. Although the tin-iron alloy has a higher electrode potential than iron in a carbonated soft drink such as Coca-Cola (trademark), the potential difference therebetween is smaller than that between chromium and iron, and the alloy is slightly dissolved in the type of solutions above-mentioned. Accordingly, the tin-iron alloy plated steel sheet of this type is advantageously less liable to local corrosion of iron than tin free steel which is another can making material. As the tin-iron alloy plated steel sheet is liable to rust than tinplate, however, it is usually coated with lacquer before formed to can.
When can bodies for food, beverage or aerosol, or other miscellaneous purposes are manufactured by welding from tinplate, it has been conventional practice to perform electrical resistance seam welding by pressing a pair of oppositely disposed roller electrodes against the overlapping portion of a preformed can body blank with wire electrodes placed therebetween, while supplying an PG,4 electric current thereto, as disclosed, for example, in Japanese Patent Publication No. 12890/1971 and Japanese Laid-Open Patent Specification No. 117142/1976. The wire electrodes are used to prevent the wear of the roller electrodes, their deformation (formation of a step along the edge of the overlapping portion) and their contamination by tin to thereby eliminate the necessity of any frequent change of the roller electrodes, and avoid any otherwise possible reduction in the working efficiency. The surface tin coating on the welded portion of a can body thus made, however, is decreased, since the greater part of tin on its original surface has been transferred to the wire electrodes with which the overlapping portion has been in contact during welding operation. Furthermore, iron is exposed on the cut edge of the overlapping portion which is not originally coated with tin, and on the extruded weld portion formed from the cut edge, i.e., the substrate portion extruded circumferentially outwardly of the cut edge by the pressure applied during welding operation. A blue or black iron oxide surface is formed if welding is done in the presence of air, while the exposed surface is composed substantially of pure iron if nonoxidizing welding is conducted in an inert gas atmosphere. Accordingly, the welded side seam of the can body is extremely inferior to the other portion thereof in corrosion resistance.
A tin-iron alloy plated steel sheet has poorer workability in electrical resistance welding than tinplate, apparently because its tin-iron alloy layer has a higher degree of electrical insulation than a layer of metallic tin, and is too hard to fit the welding electrodes tightly. In applying a conventional method for electrical resistance seam welding, a splash of molten iron is likely to scatter and adhere around the welded portion, and to cause iron exposure thereon. Such exposed iron is very difficult to be covered completely by reparing materials such as a lacquer, a plastic powder or film. Moreover, a cavity or void is likely to form in the welded portion, and thus, it is extremely difficult to obtain an excellent welded can which can preserve contents. In this connection, it may be thought to remove the tin-iron alloy by a mechanical means such as a milling cutter, or chemical dissolution from the marginal portions of a can body blank to be overlapped on each other before welding. The removal of the alloy, however, exposes iron in the welded portion of the can body and lowers its corrosion resistance, while at the same time, it naturally leads to an increase in the cost of manufacture. Even if a generally good weld may be obtained under limited welding conditions without removal of any tin-iron alloy from the marginal portions of the can body blank, iron is still exposed on the substrate portion extruded from the cut edge of the marginal portions to be welded together, and on the remaining cut edge surface. Therefore, the welded portion of the can body formed from a tin-iron alloy plated steel sheet is inferior to the other portion thereof in corrosion resistance, as in the case of the tinplate can body.